LIFEPO4/CARBON CATHODE MATERIALS OF THE LITHIUM SECONDARY BATTERY PREPARED BY ULTRASONIC SPRAY PYROLYSIS

• Main Authors: Tsung-Hsien Teng, 鄧宗憲
• Other Authors: Mu-Rong Yang
• Format: Others
• Language: en_US
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/59706724091240518951

博士 === 大同大學 === 材料工程學系(所) === 97 === Small crystalline pure LiFePO4 and doped-LiFePO4 powders with conducting carbon coating can be synthesized by ultrasonic spray pyrolysis. Cheaper trivalent iron ion is used as the precursor. The powders can be prepared with the duplex process of spray pyrolysis and subsequent heat treatment. From SEM observation, it can be found that the powders are covered with fine pyrolyzed carbon. The evenly-distributed carbon will provide intimate contact among particles to reduce the electrical resistance. Raman spectra indicate that the phase of carbon with higher electrically-conductive phase is predominant when prolonged subsequent heat treatment is carried out. On the other hand, the spherical, dense and solid LiFe(1-x)VxPO4/C (x = 0, 0.01, 0.05, 0.1) composite cathodes were prepared by introducing the polyvinyl alcohol (carbon precursor) in the precursor solution The amount of carbon in LiFe0.95V0.05PO4/C powders is 3.54 wt.% and its electrical conductivity is 2.59 × 10-2 S cm-1. The solid spherical structure provides excellent electrical contact between particles to enhance the discharge capacity and mitigate the fading rate. In contrast to the hollow spherical morphology, the solid spherical morphology of the LiFePO4/C powders will have higher discharge capacity. The cells were tested at C/10 and 1C discharge rate at 30oC. The LiFe0.95V0.05PO4/C sample, prepared at Tsp = 450oC and post heat-treatment 700oC for 8h, exhibits higher specific discharge capacity (122 mAhg−1) and no obvious fading rate after 30 charge-discharge cycles at 1C rate. The kinetics of chemical behavior of the LiFe0.97V0.03PO4/carbon cathode materials were measured by using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The chemical diffusion coefficient of lithium was found by CV and EIS were 1.14 x10-11 cm2s-1 and 1.32 x10-11 cm2s-1, respectively. The phase purity and morphology of LiFe0.97V0.03PO4 were also identified using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). The lower fading rate at high discharge rate can be attributed to the intrinsic doping, which can enhance the diffusion coefficient of Li ions and reduce the Warburg impedance and the charge-transfer resistance.